Spinal muscular atrophy is a common, recessively inherited, pediatric neuromuscular disorder caused by mutations in the Survival of Motor Neuron 1 (SMN1) gene and a deficiency of the SMN protein. SMN is ubiquitously expressed and reported to play a critical role in RNA processing, by orchestrating the biogenesis of spliceosomal small nuclear ribonucleoprotein (snRNP) particles. The assembly of these particles is severely compromised in SMA model mice. Restoring SMN to the mutants not only corrects this defect but also fully rescues the SMA phenotype. Nevertheless, SMN?s role in snRNP assembly, which is a requirement of all cells, has been difficult to reconcile with the selective neuromuscular disease phenotype characteristic of SMA. One way to explain this conundrum is to suggest that transcripts selectively expressed in one or more cells of the neuromuscular system fail to be properly processed owing to defects in SMN?s housekeeping function. Alternatively, the selective SMA phenotype could stem from novel SMN functions in the motor unit. In this project we wish to address each possibility. In aim 1 of the project we will determine if neuronal agrin, which was found to be mis-spliced in SMA motor neurons, presumably as a consequence of defects in snRNP biogenesis, is a true mediator of the SMA phenotype. Neuronal agrin is known to be important for the development of neuromuscular synapses, structures that are profoundly affected in SMA. To test possible links between agrin and the SMA phenotype, we will transgenically restore the protein selectively to the motor neurons of SMA model mice. We will then assess the consequences of agrin repletion in the mice at the molecular, cellular and phenotypic levels. In aim 2 of the project we will identify transcriptional/splice alterations in SMA motor neurons during a critical window of time that defines neuromuscular synapse maturation. This experiment takes advantage of a novel line of tamoxifen-induced SMN knockdown mice that we have developed, and exploits new findings suggesting that the requirements for the SMN protein are greatest when neuromuscular synapses mature. Following acute depletion of SMN prior to or immediately after neuromuscular synapses mature, we will catalogue motor neuronal gene expression changes in mutants and controls. This approach which complements Aim 1, but is unbiased with respect to any one gene, will uncover molecules that are important in the maturation of the neuromuscular synapses, a process that is disrupted in SMA. Some of these molecular alterations may eventually point to novel, disease-relevant and phenotype- specific functions of the protein. The collective results of the project will lead to new insights into a disease for which an optimal treatment has yet to be developed, and whose phenotype continues to puzzle scientists in light of what is currently known about the SMN protein.